Method and apparatus for modulation of a voltage controlled oscillator

ABSTRACT

A method and apparatus for modulation of a voltage-controlled oscillator (VCO). The VCO receives a tuning control voltage for adjusting a center frequency of an output periodic signal formed by the VCO. In addition, the VCO receives a modulation control voltage for modulating the output periodic signal by a content-carrying signal according to frequency modulation techniques. A frequency deviation obtained in the output periodic signal in response to changes in the modulation control voltage is linearized by forming the modulation control voltage as the result of a linear correction polynomial. A linear correction circuit forms the modulation control voltage. More particularly, a first amplifier having a gain of K 1  receives the tuning control voltage. A first summing block then receives the output of the first amplifier and adds the constant K 0 . A second amplifier having a gain of m receives the content-carrying signal. An output of the second amplifier and an output of the first summing block are summed by a second summing block. An output of the second summing block forms the modulation control voltage. The present invention results in a more linear relationship between changes in a modulation control voltage applied to the VCO and frequency deviation in the VCO output signal in comparison to prior techniques.

FIELD OF THE INVENTION

The present invention relates to the field of voltage-controlled oscillators (VCOs). More particularly, the present invention relates to the field of modulation of the operating frequency of a voltage controlled oscillator.

BACKGROUND OF THE INVENTION

A voltage-controlled oscillator (VCO) forms a periodic output signal where a frequency of the periodic output signal is related to the level of an input control voltage. The center frequency of a VCO is defined as the frequency of the periodic output signal formed by the VCO when the input control voltage is set to a nominal level, such as zero volts. The input control voltage is then adjusted up or down to control the frequency of the periodic output signal.

VCOs are used in variety of applications, such as for transmitting information according to frequency modulation techniques. To implement frequency modulation using a VCO, the center frequency is set equal to a carrier frequency to be utilized for transmitting the information. The input control voltage is then adjusted up or down in response to an information-carrying signal, thus, forming a frequency modulated signal. The frequency modulated signal is transmitted via a transmission medium to a receiver tuned to the carrier frequency. The information-carrying signal is then recovered by demodulating the received signal.

FIG. 1 illustrates a schematic diagram of a conventional VCO including a cross-coupled transistor pair. A first terminal of an inductor L₁ and a first terminal of an inductor L₂ are coupled to a supply voltage node V_(CC). A second terminal of the inductor L₁ forms a node N₁ and is coupled to a collector of a transistor Q₁, to a base of a transistor Q₂, to a cathode of a varactor diode D₁, and to a cathode of a varactor diode D₂. A second terminal of the inductor L₂ forms a node N₂ and is coupled to a collector of the transistor Q₂, to a base of the transistor Q₁, to a cathode of a varactor diode D₃, and to a cathode of a varactor diode D₄.

An anode of the varactor D₁ and an anode of the varactor D₃ are coupled to a first terminal of a resistor R₁. A second terminal of the resistor R₁ is coupled to receive a tuning control voltage V_(tune1). An anode of the varactor D₂ and an anode of the varactor D₄ are coupled to a first terminal of a resistor R₂. A second terminal of the resistor R₂ is coupled to receive a modulation control voltage V_(mod1). An emitter of the transistor Q₁ and an emitter of the transistor Q₂ are coupled to a first terminal of a resistor R₃. A second terminal of the resistor R₃ is coupled to the ground node.

In operation, an output voltage signal V_(out1) formed across the nodes N₁ and N₂ is generally a sinusoid which oscillates at the resonant frequency of the VCO. When the node N₁ is at a higher voltage level than the level of the node N₂, the transistor Q₂ has a higher bias voltage than the transistor Q₁. Accordingly, nearly all of the current through the resistor R₃ passes through the right side of the VCO (through the inductor L₂ and the transistor Q₂). This tends to reinforce the voltage at the node N₁ being higher than the voltage at the node N₂. Accordingly, this results in positive feedback in the VCO.

Eventually, however, because there is little or no current passing through the inductor L₁ and the transistor Q₁, the voltage at the node N₂ tends to rise relative to the level at the node N₁. In response, the bias on the transistor Q₁ increases while the bias on the transistor Q₂ decreases. This reduces the current in the right side of the VCO and increases the current in the left side (through the inductor L₁ and the transistor Q₁). Eventually, nearly all of the current through the resistor R₃ passes through the left side which reinforces the voltage at the node N₂ being higher than the voltage at the node N₁, through positive feedback.

Because there is little or no current passing through the right side of the VCO, the voltage at the node N₁ tends to rise relative to the level at the node N₂. In response, the bias on the transistor Q₂ increases while the bias on the transistor Q₁ decreases. Accordingly, the above-described cycle repeats. In this manner, current is alternately steered through the right and left sides of the VCO, thereby forming a the output sinusoidal signal V_(out1) across the nodes N₁ and N₂.

The tuning control voltage V_(tune1) is typically adjusted such that the output periodic signal V_(out1) oscillates at the desired center frequency when the modulation control voltage V_(mod1) is at a nominal level. The modulation control voltage V_(mod1) is then adjusted up or down to control the frequency of the output periodic signal V_(out1). A drawback to the conventional VCO illustrated in FIG. 1 is that the frequency deviation obtained in the output signal V_(out1) depends upon capacitance of each of the varactors D₁-D₄ and the amplitude of the modulation control voltage V_(mod1). As the capacitance of the varactors D₁ and D₃ increases in response to adjusting the tuning control voltage V_(tune1), the frequency deviation obtained at the output signal V_(out1) for a given modulation control voltage V_(mod1) level is reduced. Conversely, as the capacitance of the varactors D₁ and D₃ is reduced in response to adjusting the tuning control voltage V_(tune1), the frequency deviation obtained at the output V_(out1) for a given modulation control voltage V_(mod1) level increases. As a result, the VCO exhibits undesired, non-linear behavior.

FIG. 2 illustrates an exemplary graph of tuning control voltage V_(tune1) vs. frequency deviation in the output V_(out1) in response to changes in the modulation control voltage V_(mod1) for the VCO illustrated in FIG. 1. Thus, when the tuning control voltage V_(tune1) is at a level given on the x-axis, and the frequency deviation which results from an incremental (e.g., one millivolt) change in the modulation control voltage V_(mod1) is given on the y-axis. As can be seen from FIG. 2, the frequency deviation changes in a non-linear fashion with changes in the tuning control voltage V_(tune1). Difficulties can be encountered when attempting to demodulate a signal which has been modulated by a VCO which exhibits such a non-linear characteristic.

Therefore, what is needed is a technique for obtaining a more linear relationship between a tuning control voltage and frequency deviation in a VCO output signal resulting from changes in a modulation control voltage applied to the VCO.

SUMMARY OF THE INVENTION

The invention is a method and apparatus for modulation of a voltage-controlled oscillator (VCO). The VCO receives a tuning control voltage for adjusting a center frequency of an output periodic signal formed by the VCO. In addition, the VCO receives a modulation control voltage for modulating the output periodic signal by a content-carrying signal according to frequency modulation techniques. A frequency deviation obtained in the output periodic signal in response to changes in the modulation control voltage is linearized by forming the modulation control voltage as the result of a linear correction polynomial. The linear correction polynomial is preferably of the form:

V_(mod2)=K₀+(K₁)(V_(tune2))+m(V_(sig))

where V_(mod2) is the modulation control voltage, V_(tune2) is the tuning control voltage, V_(sig) is the content carrying signal and K₀, K₁ and m are constants. Appropriate values for the constants K₀, and K₁ can be determined by measuring the voltage level for V_(mod2) required to obtain a desired frequency deviation at various values of the tuning control voltage V_(tune2) (e.g., two endpoints and a center value) and with an assumed value of m(V_(sig)). Such measurements produce a numerical function which can be approximated with an interpolating polynomial by selecting the values for K₀ and K₁. The value of m can be selected to achieve a desired proportionality between frequency deviation and amplitude of the content-carrying signal V_(sig).

A linear correction circuit forms the modulation control voltage. More particularly, a first amplifier having a gain of K₁ receives the tuning control voltage (V_(tune2)). A first summing block then receives the output of the first amplifier and adds the constant K₀. A second amplifier having a gain of m receives the content-carrying signal (V_(sig)). An output of the second amplifier and an output of the first summing block are summed by a second summing block. An output of the second summing block forms the modulation control voltage (V_(mod2)).

The present invention results in a more linear relationship, in comparison to prior techniques, between a tuning control voltage and frequency deviation in a VCO output signal resulting from changes in a modulation control voltage applied to the VCO.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a conventional VCO including a cross-coupled transistor pair.

FIG. 2 illustrates an exemplary graph of tuning control voltage V_(tune1) vs. frequency deviation in the output V_(out1) in response to changes in the modulation control voltage V_(mod1) for the VCO illustrated in FIG. 1.

FIG. 3 illustrates a schematic diagram of a VCO including a linear correction circuit in accordance with the present invention.

FIG. 4 illustrates an exemplary graph of tuning control voltage V_(tune2) vs. frequency deviation in the output V_(out2) in response to changes in the modulation control voltage V_(mod2) for the VCO illustrated in FIG. 3.

FIG. 5 illustrates a schematic diagram of a VCO including a linear correction circuit in accordance with an alternate embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 3 illustrates a schematic diagram of a VCO including a linear correction circuit in accordance with the present invention. A first terminal of an inductor L₃ and a first terminal of an inductor L₄ are coupled to a supply voltage node V_(CC). A second terminal of the inductor L₃ forms a node N₃ and is coupled to a collector of a transistor Q₃, to a base (control terminal) of a transistor Q₄, to a cathode of a varactor diode D₅, and to a cathode of a varactor diode D₆. A second terminal of the inductor L₄ forms a node N₄ and is coupled to a collector of the transistor Q₄, to a base (control terminal) of the transistor Q₃, to a cathode of a varactor diode D₇, and to a cathode of a varactor diode D₈.

An anode of the varactor D₅ and an anode of the varactor D₇ are coupled to a first terminal of a resistor R₄. A second terminal of the resistor R₄ is coupled to receive a tuning control voltage V_(tune2). An anode of the varactor D₆ and an anode of the varactor D₈ are coupled to a first terminal of a resistor R₅. A second terminal of the resistor R₅ is coupled to receive a modulation control voltage V_(mod2). An emitter (output terminal) of the transistor Q₃ and an emitter (output terminal) of the transistor Q₄ are coupled to a first terminal of a resistor R₆. A second terminal of the resistor R₆ is coupled to the ground node.

In operation, an output voltage signal V_(out2) formed across the nodes N₃ and N₄ is generally a sinusoid which oscillates at the resonant frequency of the VCO. When the node N₃ is at a higher voltage level than the level of the node N₄, the transistor Q₄ has a higher bias voltage than the transistor Q₃. Accordingly, nearly all of the current through the resistor R₆ passes through the right side of the VCO (through the inductor L₄ and the transistor Q₄). This tends to reinforce the voltage at the node N₃ being higher than the voltage at the node N₄. Accordingly, this results in positive feedback in the VCO.

Eventually, however, because there is little or no current passing through the inductor L₃ and the transistor Q₃, the voltage at the node N₄ tends to rise relative to the level at the node N₃. In response, the bias on the transistor Q₃ increases while the bias on the transistor Q₄ decreases. This reduces the current in the right side of the VCO and increases the current in the left side (through the inductor L₃ and the transistor Q₃). Eventually, nearly all of the current through the resistor R₆ passes through the left side which reinforces the voltage at the node N₄ being higher than the voltage at the node N₃, through positive feedback.

Because these is little or no current passing through the right side of the VCO, the voltage at the node N₃ tends to rise relative to the level at the node N₄. In response, the bias on the transistor Q₄ increases while the bias on the transistor Q₃ decreases. Accordingly, the above-described cycle repeats. In this manner, current is alternately steered through the right and left sides of the VCO, thereby forming a the output sinusoidal signal V_(out2) across the nodes N₃ and N₄.

Together, the inductors L₃ and L₄, and the varactors D₅-D₈, form an L-C portion of the VCO. Variable capacitance for the L-C portion is provided by the varactors D₅-D₇, however, it will be apparent that one or more discrete capacitors (e.g., coupled in series or in parrallel with the inductors L₃ or L₄) can also be utilized to provide a fixed capacitance for the L-C portion of the VCO. An output periodic signal V_(out2) is obtained across the nodes N₃ and N₄. The frequency of the output signal V_(out2) is dependent upon the capacitance of the L-C portion of the VCO.

A linear correction circuit 100 forms the modulation control voltage V_(mod2) from a content-carrying signal V_(sig) and the tuning control voltage V_(tune2). More particularly, an amplifier U₁ having a gain of K₁ receives the tuning control voltage (V_(tune2)). A first summing block U₂ then receives the output of the amplifier U₁ and adds the constant K₀. An amplifier U₃ having a gain of m receives the content-carrying signal (V_(sig)). An output of the amplifier U₃ and an output of the first summing block U₂ are summed by a second summing block U₄. An output of the second summing block U₄ forms the modulation control voltage (V_(mod2)). Though the elements U₁-U₄ of the correction circuit 100 are illustrated as discrete elements, it will be apparent that the elements U₁-U₄ can be integrated and their functions combined. In addition, it will be apparent that the summing blocks U₂ and U₄ can be implemented simply as nodes.

A frequency deviation obtained in the output periodic signal V_(out2) in response to changes in the modulation control voltage V_(mod2) is linearized by the correction circuit 100 forming the modulation control voltage V_(mod2) as the result of a linear correction polynomial. The linear correction polynomial is preferably of the form:

V_(mod2)=K₀+(K₁)(V_(tune2))+m(V_(sig))

where V_(mod2) is the modulation control voltage, V_(tune2) is the tuning control voltage, V_(sig) is the content carrying signal and K₀, K₁ and m are constants. Appropriate values for the constants K₀, and K₁ can be determined by measuring the voltage level for V_(mod2) required to obtain a desired frequency deviation at various values of the tuning control voltage V_(tune2) (e.g., two endpoints and a center value) and with an assumed value of m(V_(sig)). Such measurements produce a numerical function which can be approximated with an interpolating polynomial by selecting the values for K₀ and K₁. The value of m can be selected to achieve a desired proportionality between frequency deviation and amplitude of V_(sig).

As a result of the correction polynomial implemented by the correction circuit 100, a more linear relationship is obtained between the tuning control voltage V_(tune2) and frequency deviation in a VCO output signal resulting from changes in a modulation control voltage V_(mod2) applied to the VCO.

In the preferred embodiment, the correction polynomial is first order of V_(tune2) as described above. Simulations of the invention suggest that a first order correction polynomial will correct a deviation variation measured at −20%, +25% without correction to −2.8%, +2.4% with correction over a 5:1 variation in V_(tune2) at a fixed temperature. It will be apparent, however, that the correction polynomial implemented by the correction circuit 100 can be an arbitrary order of V_(tune2). As the order of the correction polynomial increases, the linearity of the frequency deviation response also tends to increase. Higher order correction polynomials, however, will tend to improve the linearity only to a certain extent because temperature and process variations will eventually dominate the results.

FIG. 4 illustrates an exemplary graph of offset voltage V_(offset2) vs. frequency deviation in the output V_(out2) for the VCO illustrated in FIG. 3. Thus, when the tuning control voltage V_(tune2) is at a level given on the x-axis, and the frequency deviation which results from an incremental (e.g., one millivolt) change in the modulation control voltage V_(mod2) is given on the y-axis. As can be seen from FIG. 4, the frequency deviation is substantially constant despite changes in the tuning control voltage V_(tune2). This improves results obtained when demodulating a signal which has been modulated by a VCO which exhibits such a linear characteristic.

FIG. 5 illustrates a schematic diagram of a VCO including a linear correction circuit in accordance with an alternate embodiment of the present invention. The VCO illustrated in FIG. 5 differs from that illustrated in FIG. 3 in that the resistor R₆ of FIG. 3 omitted and the emitters of the transistors Q₃ and Q₄ are no longer coupled together. Instead, in FIG. 5, a capacitor C₁ is coupled across the emitters of the transistors Q₃ and Q₄, a first terminal of a resistor R₇ is coupled to the emitter of Q₃ and a first terminal of a resistor R₈ is coupled to the emitter of Q₄. A second terminal of the resistor R₇ and a second terminal of the resistor R₈ are coupled to the ground node.

The capacitor C₁ coupled across the emitters of the cross-coupled pair of transistors Q₃ and Q₄ in FIG. 5 counteracts the effects of phase lag caused by the transistors Q₃ and Q₄. Accordingly, this arrangement provides improved performance in that it results in a higher small-signal loop gain at the zero phase crossing which provides reliable start-up; has a peak gain which is closer to a zero phase crossing, which reduces frequency drift during settling; and has a low output susceptance. Also, a periodic output signal formed by the VCO can be taken at the emitters of the cross-coupled pair Q₃ and Q₄ (across the capacitor C₁), which avoids having to utilize a buffer which could increase phase noise.

The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention. Specifically, it will be apparent to one of ordinary skill in the art that the device of the present invention could be implemented in several different ways and the apparatus disclosed above is only illustrative of the preferred embodiment of the invention and is in no way a limitation. For example, it would be within the scope of the invention to vary the values of the various components, current levels, and voltage levels disclosed herein. Further, it would be within the scope of the invention to substitute another type of transistor, such as a field effect transistor, for any of the transistors disclosed herein. In addition, though the preferred VCO includes a cross-coupled pair of transistors, it will be apparent that the VCO be of another topology. 

What is claimed is:
 1. An apparatus for linearizing a frequency deviation response for a voltage controlled oscillator, the apparatus comprising: a. a voltage-controlled oscillator having a tuning input to receive a tuning control signal to adjust a center frequency for the voltage-controlled oscillator and a modulation input to receive a modulation control signal to modulate an output signal formed by the voltage-controlled oscillator; and b. a correction circuit coupled to the voltage-controlled oscillator to receive the tuning control signal and a content-carrying signal to form the modulating control signal based upon the tuning control signal and the content-carrying signal.
 2. The apparatus according to claim 1 wherein the correction circuit forms a first order polynomial.
 3. The apparatus according to claim 1 wherein the correction circuit forms a polynomial of the form: V_(mod2)=K₀+(K₁)(V_(tune2))+m(V_(sig)) where V_(mod2) is the modulation control voltage, V_(tune2) is the tuning control voltage, V_(sig) is the content carrying signal and K₀, K₁ and m are constants.
 4. The apparatus according to claim 1 wherein the correction circuit comprises: a. a first amplifier having a constant gain coupled to receive the tuning control voltage for forming a first amplifier output; b. a first summing block coupled to receive the first amplifier output and a constant for forming a first summing block output; c. a second amplifier having a constant gain coupled to receive the content-carrying signal for forming a second amplifier output; and d. a second summing block coupled to receive the first summing block output and the second amplifier output for forming the modulation control signal.
 5. The apparatus according to claim 1 wherein the voltage-controlled oscillator comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a resistor having a first terminal coupled to output terminals of the cross-coupled pair of transistors and a second terminal coupled to a ground node.
 6. The apparatus according to claim 1 wherein the voltage-controlled oscillator comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a capacitor coupled across output terminals of the cross-coupled pair of transistors; and d. a pair of resistors each having a first terminal coupled to a respective one the output terminals of the cross-coupled pair of transistors and each having a second terminal coupled to a ground node.
 7. An apparatus for linearizing a frequency deviation response for a voltage controlled oscillator, the apparatus comprising: a. oscillator means having a tuning input for receiving a tuning control signal for adjusting a center frequency for the oscillator means and a modulation input for receiving a modulation control signal for modulating an output signal formed by the oscillator means; and b. means for forming the modulating control signal based upon the tuning control signal and a content-carrying signal wherein the means for forming is coupled to the oscillator means.
 8. The apparatus according to claim 7 wherein the means for forming forms a first order polynomial.
 9. The apparatus according to claim 7 wherein the means for forming forms a polynomial of the form: V_(mod2)=K₀+(K₁)(V_(tune2))+m(V_(sig)) where V_(mod2) is the modulation control voltage, V_(tune2) is the tuning control voltage, V_(sig) is the content carrying signal and K₀, K₁ and m are constants.
 10. The apparatus according to claim 7 wherein the means for forming comprises: a. a first amplifier having a constant gain coupled to receive the tuning control voltage for forming a first amplifier output; b. a first summing block coupled to receive the first amplifier output and a constant for forming a first summing block output; c. a second amplifier having a constant gain coupled to receive the content-carrying signal for forming a second amplifier output; and d. a second summing block coupled to receive the first summing block output and the second amplifier output for forming the modulation control signal.
 11. The apparatus according to claim 7 wherein the oscillator means comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a resistor having a first terminal coupled to output terminals of the cross-coupled pair of transistors and a second terminal coupled to a ground node.
 12. The apparatus according to claim 7 wherein the oscillator means comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a capacitor coupled across output terminals of the cross-coupled pair of transistors; and d. a pair of resistors each having a first terminal coupled to a respective one the output terminals of the cross-coupled pair of transistors and each having a second terminal coupled to a ground node.
 13. An apparatus for linearizing a frequency deviation response for a voltage controlled oscillator, the apparatus comprising: a. a voltage-controlled oscillator having a tuning input to receive a tuning control signal to adjust a center frequency for the voltage-controlled oscillator and a modulation input to receive a modulation control signal to modulate an output signal formed by the voltage-controlled oscillator; and b. a correction circuit coupled to the voltage-controlled oscillator to receive the tuning control signal and a content-carrying signal to form the modulating control signal such that a frequency deviation caused by an incremental change in the modulating control signal is substantially constant despite changes in the tuning control voltage.
 14. The apparatus according to claim 13 wherein the correction circuit forms a first order polynomial.
 15. The apparatus according to claim 13 wherein the correction circuit forms a polynomial of the form:  V_(mod2)=K₀+(K₁)(V_(tune2))+m(V_(sig)) where V_(mod2) is the modulation control voltage, V_(tune2) is the tuning control voltage, V_(sig) is the content carrying signal and K₀, K₁ and m are constants.
 16. The apparatus according to claim 13 wherein the correction circuit comprises: a. a first amplifier having a constant gain coupled to receive the tuning control voltage for forming a first amplifier output; b. a first summing block coupled to receive the first amplifier output and a constant for forming a first summing block output; c. a second amplifier having a constant gain coupled to receive the content-carrying signal for forming a second amplifier output; and d. a second summing block coupled to receive the first summing block output and the second amplifier output for forming the modulation control signal.
 17. The apparatus according to claim 13 wherein the voltage-controlled oscillator comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a resistor having a first terminal coupled to output terminals of the cross-coupled pair of transistors and a second terminal coupled to a ground node.
 18. The apparatus according to claim 13 wherein the voltage-controlled oscillator comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a capacitor coupled across output terminals of the cross-coupled pair of transistors; and d. a pair of resistors each having a first terminal coupled to a respective one the output terminals of the cross-coupled pair of transistors and each having a second terminal coupled to a ground node.
 19. A method of linearizing a frequency deviation response in a voltage controlled oscillator, the method comprising: a. providing an oscillator means having a tuning input for receiving a tuning control signal for adjusting a center frequency for the oscillator means and a modulation input for receiving a modulation control signal for modulating an output signal formed by the oscillator means; and b. forming the modulating control signal based upon the tuning control signal and a content-carrying signal.
 20. The method according to claim 19 wherein the step of forming forms a first order polynomial.
 21. The method according to claim 19 wherein the step of forming forms a polynomial of the form: V_(mod2)=K₀+(K₁)(V_(tune2))+m(V_(sig)) where V_(mod2) is the modulation control voltage, V_(tune2) is the tuning control voltage, V_(sig) is the content carrying signal and K₀, K₁ and m are constants.
 22. The method according to claim 19 wherein the step of forming is performed by a correction circuit comprising: a. a first amplifier having a constant gain coupled to receive the tuning control voltage for forming a first amplifier output; b. a first summing block coupled to receive the first amplifier output and a constant for forming a first summing block output; c. a second amplifier having a constant gain coupled to receive the content-carrying signal for forming a second amplifier output; and d. a second summing block coupled to receive the first summing block output and the second amplifier output for forming the modulation control signal.
 23. The method according to claim 19 wherein the oscillator means comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a resistor having a first terminal coupled to output terminals of the cross-coupled pair of transistors and a second terminal coupled to a ground node.
 24. The method according to claim 19 wherein the voltage-controlled oscillator comprises: a. an L-C portion wherein capacitance for the L-C portion is variable in response to the tuning control signal and in response to the modulation control signal; b. a pair of cross-coupled pair of transistors, each having a control terminal coupled to the L-C portion; c. a capacitor coupled across output terminals of the cross-coupled pair of transistors; and d. a pair of resistors each having a first terminal coupled to a respective one the output terminals of the cross-coupled pair of transistors and each having a second terminal coupled to a ground node. 